Water-soluble packaging material commonly comprises polyvinyl alcohol films. Polyvinyl alcohol is manufactured commercially by polymerization of vinyl acetate monomer to yield polyvinyl acetate. The polyvinyl acetate is then transesterified, typically using methanol, to yield polyvinyl alcohol and methyl acetate. Polyvinyl alcohol that is greater than 98% hydrolyzed (that is, less than 2% residual vinyl acetate) is unacceptable for water treatment applications because it does not dissolve at practical temperatures, usually requiring temperatures in excess of 50° C. to dissolve. Of greater utility are packaging materials that dissolve in cold water, that is, water at a temperature of from 10-25° C. This can be accomplished by modifying polyvinyl alcohol to reduce crystallinity and/or increase hydrophilicity of the polymer. The crystallinity of the polymer is conveniently and conventionally reduced by carrying out the transesterification of polyvinyl acetate in such a manner as to not complete the conversion to polyvinyl alcohol and obtain a product that is conventionally known as partially hydrolyzed polyvinyl alcohol.
In order to be practical and useful, the films should be compatible with the chemicals they contain. For example, a film made from partially hydrolyzed polyvinyl alcohol is sensitive to high or low pH chemicals, as they cause hydrolysis of the residual vinyl acetate, causing the film composition to change towards the cold-water insoluble and fully hydrolyzed polyvinyl alcohol. Acidic pool chemicals such as potassium hydrogen peroxymonosulfate or caustic pool chemicals, if packaged in a film comprising partially hydrolyzed polyvinyl alcohol, cause the hydrolysis reaction which results in a deterioration of the film solubility. Additionally the packages can exhibit an odor of acetic acid, which is unpleasant to consumers.
Buckland et al., in U.S. Pat. No. 6,727,219, disclose stabilized potassium hydrogen peroxymonosulfate packaged in a sealed water-soluble pouch, optionally including various additives, including pH buffers, such as anhydrous sodium carbonate and bicarbonate; active halogen agents such as halogenated hydantoins; and halogen stabilizers such as cyanuric acid. The water-soluble pouch compositions included partially hydrolyzed polyvinyl alcohol. Buckland et al. do not provide information on the stability of partially hydrolyzed polyvinyl alcohol films to long-term storage in contact with stabilized potassium hydrogen peroxymonosulfate compositions. Such storage causes reduced film solubility and formation of acetic acid.
In U.S. Pat. No. 6,821,590, Verrall et al. describe starch-loaded polyvinyl alcohol copolymer films for packaging non-liquid products. Starch loadings were from about 4 to about 30% by weight. This patent does not teach how to package low pH solids.
It is desirable to have a storage stable single dosage oxidizing agent for treatment of recreational waters. Such an agent requires that a stable pouch material be used to package such moisture-sensitive acidic materials to avoid loss of cold-water solubility, acetic acid formation, and other indicators of deterioration on prolonged storage. The present invention provides such a product.